Prediction of global influenza seasons during post-covid-19 pandemic

Abstract

Background: Various non-pharmaceutical interventions (NPIs) were implemented globally to mitigate COVID-19, which could have modulated the transmission dynamics of directly transmitted viruses including influenza. Further, over time the susceptibility for influenza in community could have significantly increased. We aim to first assess the impact of COVID-19 NPIs on the transmissibility influenza viruses and then predict the forthcoming seasonal influenza epidemics globally. Methods: We used the surveillance data on influenza virus activity, available for different countries during 2016-2021. We first evaluate the comparable proxy of the influenza activity for each country and estimated transmissibility as instantaneous reproduction number. Then we assess the impact of the NPIs by evaluating the reduction in transmissibility as well as infections by using regression and state-space models and compared with the previous years. Finally, we developed a data-driven simulation based mechanistic predictive model framework to infer the forthcoming influenza epidemic incorporating available information on intrinsic and extrinsic drivers in the models. Results: We estimated 20% to 42% reduction in transmissibility and 5% to 25% reduction in infections of influenza by the NPIs with variations across the countries based on the respective NPIs implemented. We estimated the threshold of the impact strength of NPIs as 10%-15% reductions in the transmissibility could be sufficient to prevent infections from spreading, hence suppress summer epidemic in sub-tropical locations like Hong Kong. We predicted that the upcoming influenza epidemic will be increased by 2-4 folds in infections under several possible scenarios on timing of the next season and the wining of the effect of COVID-19 NPIs. We estimated a 18% - 45% vaccination coverage for seasonal influenza strains before the start of the season may able to avoid this excess influenza infections in these locations. Conclusions: Our findings indicate the evidence of the indirect impact of COVID-19 NPIs on mitigating influenza transmission globally. It is important that medical and healthcare settings are prepared for large future influenza seasons across the countries given the loss of immunity to influenza in since early 2020.